Lithium inclusion in indium metal-organic frameworks showing increased surface
area and hydrogen adsorption
Mathieu Bosch, Muwei Zhang, Dawei Feng, Shuai Yuan, Xuan Wang, Ying-Pin Chen, and Hong-Cai Zhou
Citation: APL Materials 2, 124103 (2014); doi: 10.1063/1.4897475
View online: http://dx.doi.org/10.1063/1.4897475
View Table of Contents: http://scitation.aip.org/content/aip/journal/aplmater/2/12?ver=pdfcov
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APL MATERIALS 2, 124103 (2014)
Lithium inclusion in indium metal-organic frameworks
showing increased surface area and hydrogen adsorption
Mathieu Bosch,1 Muwei Zhang,1 Dawei Feng,1 Shuai Yuan,1 Xuan Wang,1
Ying-Pin Chen,2 and Hong-Cai Zhou1,2,a
1
Department of Chemistry, Texas A&M University, College Station, Texas 77842, USA
Department of Materials Science and Engineering, Texas A&M University, College Station,
Texas 77842, USA
2
(Received 4 August 2014; accepted 20 September 2014; published online 20 October 2014)
Investigation of counterion exchange in two anionic In-Metal-Organic Frameworks
(In-MOFs) showed that partial replacement of disordered ammonium cations was
achieved through the pre-synthetic addition of LiOH to the reaction mixture. This
resulted in a surface area increase of over 1600% in {Li [In(1,3-BDC)2]}n and
enhancement of the H2 uptake of approximately 275% at 80 000 Pa at 77 K. This
method resulted in frameworks with permanent lithium content after repeated solvent
exchange as confirmed by inductively coupled plasma mass spectrometry. Lithium
counterion replacement appears to increase porosity after activation through replacement of bulkier, softer counterions and demonstrates tuning of pore size and properties in MOFs. C 2014 Author(s). All article content, except where otherwise
noted, is licensed under a Creative Commons Attribution 3.0 Unported License.
[http://dx.doi.org/10.1063/1.4897475]
Metal-Organic Frameworks (MOFs) are an emerging category of porous materials that consist
of metal-containing units and organic linkers.1,2 They are promising materials for the storage of
fuel gases like methane and hydrogen, but so far they fall short of targets for gravimetric and
volumetric capacity set to make them superior to current fuel technologies.3,4 Recently, theoretical
studies have shown that lithium doping is a viable way to increase hydrogen uptake near ambient
conditions, through increased electrostatic interactions.5 Additionally, crystalline ionic MOFs are
promising materials for ion exchange and separation.6 MOFs that are easily impregnated by lithium
ions should possess several attributes.7 First, this MOF should possess considerable stability, to
retain porosity after lithium inclusion and the subsequent activation process. Second, this MOF
should be negatively charged, so that Li+ ions can be held in the framework by electrostatic forces.
Many indium MOFs possess a negatively charged framework which may stabilize the presence
of positively charged lithium ions in the pores.8–10 Furthermore, it has been hypothesized that the
breakdown of solvents into ionic molecules during the solvothermal reaction plays an important role
in MOF synthesis, particularly in ionic MOFs which require a counterion.11–13 In principle, it is
suggested that the exchange of bulkier counterions for small lithium ions should be a facile method
to increase the porosity of any MOF with an anionic framework.
In order to test the hypothesis outlined above, several synthetic methods were tried to introduce
lithium into several MOFs. In(1, 3 − BDC)−2 (BDC = 1,3-benzene dicarboxylate) and In(NDC)−2
(NDC = 2,6-naphthalene dicarboxylate) were synthesized according to the literature procedures.9,10
After the crystal synthesis, exchange reactions were performed with various lithium metal sources,
including lithium chloride, lithium acetate, and lithium nitrate. Unexpectedly, these efforts led to
either framework collapse (possibly due to exposure to atmospheric moisture) or no increase in
porosity. It is suggested that the cations in the frameworks as reported were too bulky to be removed
by a simple exchange process, and thus this approach was unsuccessful.
a Email: zhou@mail.chem.tamu.edu. Tel.: +1 979 845 4034. Fax: +1 979 845 1595.
2166-532X/2014/2(12)/124103/5
2, 124103-1
© Author(s) 2014
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124103-2
Bosch et al.
APL Mater. 2, 124103 (2014)
In our experimental studies, a direct synthesis method was used, which introduced LiOH
directly to the solvothermal reaction mixture before sonication and heating. This was intended to
bring about the immediate formation in solution of some proportion of ligand molecules that were
deprotonated to form lithium carboxylates, which would then form a framework with the lithium
ions close at hand for charge balancing during synthesis. This resulted in an In-BDC framework
with significantly increased porosity compared to the version filled with disordered tetramethyl
ammonium cations in the framework, with the greatest porosity found when a number of moles
of lithium hydroxide equal to the moles of ligand present were added before solvothermal synthesis. Based on stoichiometric control, we found that addition of LiOH in ratios higher than 1:1
LiOH:ligand produced amorphous material as shown by powder X-Ray Diffraction (PXRD). Attempts to produce MOF with added NaOH or KOH produced a non-crystalline, non-porous product,
presumably due to the drastically increased basicity of those solutions.
Both investigated compounds, {[N(CH3)4][In(1,3-BDC)2]}n and {Li[In(1,3-BDC)2]}n, possess
a very similar structure to that described in the literature9 as verified by PXRD (Figure 1), which was
that of one dimensional anionic nanotubes, with each indium connected to four isophthalates, shown
in Figure 2. Extraneous peaks were found in the PXRD measurement of {[N(CH3)4][In(1,3-BDC)2]}n
at approximately 16◦, 19◦, and 34.5◦, likely indicating some impurity accompanied the starting materials.9
As shown in Figure 1, slight downshifting of the peaks in the PXRD for {Li[In(1,3-BDC)2]}n
could indicate a slightly larger unit cell for the experimental pattern, while slight changes of the
intensity ratios could be attributed to different occupancies, with lithium replacing tetramethylammonium.14 The dimensional variation of the unit cell could be influenced by a different test
temperature—the experimental PXRDs were taken at room temperature, which should cause a
slight expansion of the unit cell compared to the reported single crystal XRD taken at 100 K.15 It
could also arise from a greater degree of solvent remaining in the crystals for the PXRD pattern,
as the PXRD was taken from sample directly removed from the solvent without activation. The
difference in peak intensities may be caused by solvent molecules and counterions in the channels
of the structure, which could not be crystallographically localized but contribute to the experimental
X-ray diffraction.16
For example, the highest observed intensity in the experimental pattern occurs on the (2 1̄ 0)
plane of the unit cell, which includes the line describing the center of the nanotube, shown in Figure 2. Therefore, a great deal of electron density is shown on this plane in the experimental pattern
which is absent in the pattern simulated from the Crystallographic Information File(CIF). This may
be because the single-crystal solution reported did not include the contents of this solvent-accessible
FIG. 1. Powder XRD patterns for simulated and experimental In(1, 3 − BDC)2 samples. The (2 1̄ 0) reflection is at roughly
12◦ 2θ.
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124103-3
Bosch et al.
APL Mater. 2, 124103 (2014)
FIG. 2. A slightly offset crystallographic view of the ab plane of {[N(CH3)4][In(1,3-BDC)2]}n, with the blue line representing
the (2 1̄ 0) plane, which continuously includes the center of the nanotube. Large white spheres correspond to carbon atoms,
small white to hydrogen, red to oxygen, and green to indium. The blue corresponds to nitrogen occupying disordered
positions.
space inside the nanotube. The large amount of electron density in the center of the nanotube shown
by the experimental pattern likely consists of molecules trapped within the nanotubes, in addition
to the oxygen and carbon atoms on the framework also intersected by the plane. It cannot be ruled
out that some slight shift in the framework structure may have occurred as a consequence of lithium
inclusion. However, the similarity of the PXRD patterns indicates that any such shift is minor.
Following solvent exchange and activation, gas sorption measurements were performed on both
materials. The N2 adsorption measurements showed that Brunauer, Emmett and Teller(BET) surface area increased from approximately 26 m2/g in {[N(CH3)4][In(1,3-BDC)2]}n to approximately
420 m2/g in {Li[In(1,3-BDC)2]}n. {Li[In(1,3-BDC)2]}n had H2 uptake of approximately 110 cm3/g at
800 millibars, while In-BDC-N(CH3)+4 showed H2 uptake of 40 cm3/g at 800 millibars, and hysteresis
in its H2 isotherm was consistent with small pore size. {Li[In(1,3-BDC)2]}n shows increased pore size
as well as increased N2 and H2 uptake. This is consistent with a hypothesized smaller Li+ counterion
in an otherwise very similar framework (Fig. 3).
After repeated washes of the material with dimethylformamide(DMF) and ethanol, and multiple cycles of overnight solvent exchange, ICP-MS measurements showed that {Li[In(1,3-BDC)2]}n
possessed an indium-lithium ratio of approximately one lithium per two indium. This provides evidence that Li+ is permanently ionically attached in the pores as a counterion, indicating significant
but incomplete counterion replacement.
FIG. 3. (Left) H2 and (right) N2 sorption isotherms for {[N(CH3)4][In(1,3-BDC)2]}n (red) and {Li[In(1,3-BDC)2]}n (blue).
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124103-4
Bosch et al.
APL Mater. 2, 124103 (2014)
While the presence of lithium in the In-BDC framework can be preliminarily confirmed by
the ICP results, the increased surface area, and hydrogen adsorption data, further crystallographic
evidence was sought. However, single-crystal XRD characterization of {Li[In(1,3-BDC)2]}n was
unsuccessful. Instead, synthesis of the In(2,6-NDC) series was undertaken to further explore the
incorporation of Li+ ions in anionic MOFs from the crystallographic perspective.
Single crystal XRD was taken for multiple crystals of {Li[In(NDC)2]}n, all of which were solved
in the P4/n space group. The reaction mixture including LiOH contained crystals with two separate
habits. Clear rods were measured which also had a framework identical to {(Et2NH2)[In(2,6-NDC)2∗
2H2O∗DEF]}n, while purple blocks were also isolated which had identical connectivity, but a unit
cell with a short axis elongated from 8.2 Å to 8.7 Å. This corresponds to a change in the indiumindium distance,10 and the pore length, of the same amount on that axis. However, the topology
of these networks remained consistent, and significant electron density next to the indium in the
clear rods found in {(Et2NH2)[In(2,6-NDC)2∗2H2O∗DEF]}n was significantly lessened in the Li+
version, consistent with Li+ ions replacing some proportion of [Et2NH2]+. This is in contrast to
{(Et2NH2)[In(2,6-NDC)2∗2H2O∗DEF]}n where the (Et2NH2)+ counterion could be crystallographically located (Fig. 4).10 We were careful to verify this before the removal of disordered solvent
electron density using the SQUEEZE routine of PLATON, which could interfere with such analysis.
ICP results also confirmed the presence of significant lithium in the pores even after washes with
DMF and ethanol, though the Li-In ratio found for this sample may not be accurate due to difficulties
in acquiring a sample including only one type of crystal.
Pure samples of {Li[In(NDC)2]}n crystals were not obtained in quantities necessary for gas
sorption measurement, as all samples prepared with LiOH contained both types of crystals. However, this provides evidence that Li+ can be successfully incorporated into multiple, topologically
dissimilar negatively charged indium MOFs through direct solvothermal synthesis in the presence
of LiOH. See supplementary material for details of synthesis, XRD, and ICP-MS measurements.17
In conclusion, we showed that partial replacement of disordered ammonium cations with
lithium ions in two anionic In-MOFs was possible through the pre-synthetic addition of LiOH to
the reaction mixture. The effects of the replacement of ammonium cations in anionic In-MOFs with
Li+ were investigated, resulting in a surface area increase of over 1600% in {Li[In(1,3-BDC)2]}n.
The H2 uptake was improved by 275% at 800 millibars. Preparation of {Li[In(1,3-BDC)2]}n yielded
a powder product of lower crystallinity than that produced by the less basic carboxylic acid ligand.
Synthesis of {Li[In(NDC)2]}n which was more easily crystallized provided additional evidence of
FIG. 4. Crystallographic view of the ab plane of the framework of {Li[In(NDC)2]}n, which consisted of crystals of a separate
color and habit than that of {(Et2NH2)[In(2,6-NDC)2∗2H2O∗DEF]}n, while possessing an identical topology.10,18
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124103-5
Bosch et al.
APL Mater. 2, 124103 (2014)
lithium inclusion through ICP-MS measurements and the presence of significantly reduced counterion electron density in the lithiated version through X-ray analysis. This synthetic approach
may be applicable to more MOFs possessing anionic frameworks, and replacement of bulky with
small and hard counterions should be accompanied by an increase in pore volume. Furthermore,
as ICP-MS indicated a molar ratio of one lithium per two indium over repeated measurements,
synthetic tuning may produce a greater degree of counterion replacement and allow higher porosity
than that reported here. This synthetic strategy also provides a facile method to tune the pore
sizes of various anionic MOFs, for example, size-selective separation of various gas molecules
through counterion exchange in topologically identical frameworks.19 Finally, this material may be
promising for investigation of its Li+ or other ion conduction properties.20
This work was supported as part of the Hydrogen and Fuel Cell Program under Award No.
DE-FC36-07GO17033 funded by the U.S. Department of Energy (DOE), Office of Science, Office
of Basic Energy Sciences. M.B. also acknowledges the Texas A&M graduate merit fellowship.
We would like to acknowledge Dr. William D. James and the Texas A&M University Center for
Chemical Characterization and Analysis for their contributions to this project.
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2
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